Identification of the methylated ribosomal proteins in HeLa cells and the fluctuation of methylation during the cell cycle.

Methylated proteins from HeLa cell cytoplasmic ribosomes have been identified. At least seven proteins are methylated and four of them are mildly acidic. The nature of the methylated amino acid in each protein is presented. In synchronized HeLa cell culture, the extent of methylation for both subunits varies with the cell cycle. Methylation of the 40 S subunit occurs heavily in the late G1 phase whereas methylation of the 60 S subunit is most pronounced in the early S phase.

Purification and properties of a ribosomal protein methylase from Eschericha coli Q13.

Ribosomal protein methylase has been purified from Escherichia coli strain Q13 using methyl-deficient 50S subunits as substrates. The purified enzyme (or enzyme complex) which is devoid of rRNA methylating activity is quite stable and has a pH optimum around 8.0. The Km for S-adenosyl-L-methionine is 3.2 muM. The molecular weight of the enzyme is 3.1 X 10(4); minor methylating activity was also detected for protein peaks with molecular weights of 1.7 X 10(4) and 5.6 X 10(4). Protein L11 is the major protein methylated by the purified enzyme. Product analysis revealed the presence of N epislon-trimethyllysine, a methylated neutral amino acid(s) previously observed in protein L11 and N epislon-monomethyllysine. Free ribosomal proteins were much better substrates for the methylation, indicating that methylation of 50S ribosomal proteins can occur before the complete assembly of the 50S ribosomal subunit.